Two material achromatic prism

ABSTRACT

A spectrometer comprises a detector array and a prism. The prism comprises a first prism element comprising a substantially crystalline crown material, and a second prism element contacting the first prism element, the second prism element comprising a substantially crystalline flint material. The spectrometer further includes optics configured to direct light at least twice through the prism. The prism is configured to disperse light received from the optics at an incident angle therethrough into constituent spectra in visible and infrared wavelength bands that are dispersed from the prism at angles offset from the incident angle. The constituent spectra are directed onto the detector array with approximately equal dispersion across the visible and infrared wavelength bands. Among other things, desirable material selections for the first and second prism elements are also disclosed.

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/896,620, filed Oct. 1, 2010, and incorporated herein in itsentirety by reference.

BACKGROUND

This disclosure relates generally to spectrally dispersive opticalsystems. More particularly, this disclosure relates to spectrometersutilizing two-material prism elements.

Numerous optical systems utilize the dispersion of light intoconstituent wavelength bands. Such dispersion is used for many purposes,including but not limited to spectrometry or other spectral analysis.The dispersion of light is conventionally achieved through a dispersiveelement, such as a diffraction grating or a prism.

Diffraction grating dispersive elements, which include singleblaze-angle or multi-faceted (i.e. dual-blaze angle) gratings, utilizethe periodic nature of the constituent material to separate outdifferent wavelength bands of light as it intersects the blazing of thegrating. Prisms, on the other hand, utilize the transition betweenmaterials, such as between air and the solid of the prism, to dispersethe incident light. Prisms generally have the advantage of yielding ahigher total throughput via their dispersion than a grating achievesthrough diffraction, particularly when operating over a spectral rangethat includes more than one octave (here an octave refers to a doublingin frequency). However, prisms may be more prone to certain distortions,as discussed below.

A prism utilizing only a single material yields a dispersion curveproportional to the change in the material's index of refraction versuswavelength. Typically this change in dispersion with wavelength is verylarge and acts as an unacceptable distortion. To counteract the unwanteddistortion of some prisms, multiple elements of different materials(conventionally different types of glass) and/or different shapes may bejoined and utilized to control the dispersion of different wavelengthbands to reach a desired optical effect. The concept of utilizingmultiple materials in prisms utilizes a high dispersion material (i.e. a“flint” glass) with a low dispersion material (i.e. a “crown” glass), toform an achromatic pair that allows control of the dispersion ofdifferent wavelengths of light, which may be optimized for particularoptical systems. For example, two-material prisms may be configured tocorrect for undesired dispersion effects in the optical system. Withprisms, one common undesired dispersion effect that may arise is a largevariation from linearity of the spectral dispersion, which may resultfrom the differing refractive indexes associated with differentwavelengths of light in the optical elements.

In various situations, desired optical system specifications may preferselection of systems that meet a particular spectral sampling interval(the spacing between sample points in a measured spectra). To achievethe desired sampling interval, the dispersive element may be configuredto limit variance in detector bandwidth over a wide band of wavelengths.Such limited variance may be characterized by the system having arelatively linear or “flat” dispersion. To achieve relative flatness ofdispersion in prism-based dispersive elements, designers may select froma large number of flint and crown materials, to construct a two-materialachromatic prism. As one example of conventional material selection,some optical systems may utilize a two-glass prism having a fused quartzcrown (an amorphous/glassy form of quartz, such as that distributedunder the trademark Infrasil®) and an SF1 flint (where SF1 is the Schottglass designation for a common flint glass). Thus far, materialselection for conventional two-material achromatic prisms forspectrometers have involved combining glassy materials based onempirical observations of their properties, and refining materialselections based on the effectiveness of the combination.

What is needed is, among other things, improvements over knowndispersive elements for spectroscopic purposes, wherein theirconstituent materials provide reduced variance of dispersion over a widewavelength band.

SUMMARY

According to an embodiment, a spectrometer comprises a detector arrayand a prism. The prism comprises a first prism element comprising asubstantially crystalline crown material, and a second prism elementcontacting the first prism element, the second prism element comprisinga substantially crystalline flint material. The spectrometer furtherincludes optics configured to direct light at least twice through theprism. The prism is configured to disperse light received from theoptics at an incident angle therethrough into a constituent spectra invisible and infrared wavelength bands, the spectra being dispersed fromthe prism at angles offset from the incident angle. The constituentspectra are directed onto the detector array with approximately equaldispersion across the visible and infrared wavelength bands.

According to another embodiment, a two-material prism comprises a firstprism element comprising Aluminum Oxynitride. The two-material prismfurther comprises a second prism element comprising Strontium Titanate.An associated face of the first prism element is contacting anassociated face of the second prism element, such that light dispersedthrough both the first prism element and the second prism element has adispersive characteristic suitable for operation in a visible toshort-wave infrared wavelength band.

According to another embodiment, a two-material prism comprising a firstprism element comprising a crown material, and a second prism elementcomprising an iodine-based ionic compound crystalline salt flintmaterial. An associated face of the first prism element is contacting anassociated face of the second prism element, such that light dispersedthrough both the first prism element and the second prism element has adispersive characteristic suitable for operation in a visible toshort-wave infrared wavelength band.

Other aspects and embodiments will become apparent from the followingdetailed description, the accompanying drawings, and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features of embodiments of this disclosure are shown in thedrawings, in which like reference numerals designate like elements.

FIG. 1 shows spectrometer of the present disclosure having atwo-material prism and optics arranged in a double pass configuration.

FIG. 2 shows an example prescription for the spectrometer of FIG. 1.

FIG. 3 shows the two-material prism of the spectrometer of FIG. 1 havinga mirror arranged in a minimum deviation (MD) configuration.

FIG. 4 shows the two-material prism of the spectrometer of FIG. 1 havinga mirror arranged in a rear surface reflection (RS) configuration.

FIG. 5 shows a plot of the refractive index versus Abbe number for avariety of crown and flint materials.

FIG. 6A shows a graph depicting relative dispersion over a band ofwavelengths for different material selections for two-material prisms,as compared to an example one-material prism.

FIG. 6B shows a re-scaled view of the graph of FIG. 6A, omitting theexample one-material prism to enhance the plot for the two-materialprisms.

FIG. 6C replots the graph of FIG. 6B, omitting those material pairs thatdo not utilize Silver Chloride as the flint material.

FIG. 7A plots relative dispersion versus wavelength for another exampleof a two-material prism configuration, using the two-materialcombinations with Silver Chloride as the flint material depicted in FIG.6C.

FIG. 7B plots relative dispersion versus wavelength for the example ofthe two material prism configuration of FIG. 7A, however utilizingdifferent two-material pairs that do not utilize Silver Chloride as theflint material.

FIG. 8 plots the bandwidth as a function of wavelength for embodimentsof conventional and Silver Chloride based two-material prisms.

FIG. 9 shows the relative dispersion over a band of wavelengths for avariety of two-material pairs, highlighting the effect of differentreflection configurations, specifically MD or RS, for the prisms.

FIG. 10 shows a plot of the refractive index versus Abbe number for avariety of ionic salt flint materials, in relation to the crown andflint materials depicted in FIG. 5.

FIG. 11 shows a plot of the relative dispersion versus wavelength fortwo-material prisms utilizing the ionic salt flints of FIG. 10, inrelation to a reference set of the crown/flint combinations depicted inFIG. 5.

FIG. 12 replots the graph of FIG. 11, omitting all of the ionic saltsexcept for Cesium Bromide, emphasizing the comparison between CesiumBromide and the reference set of crown/flint combinations.

FIG. 13 replots the graph of FIG. 11, omitting all of the ionic saltsexcept for Cesium Iodide, emphasizing the comparison between CesiumIodide and the reference set of crown/flint combinations.

FIG. 14 replots the graph of FIG. 11, omitting several of the ionicsalts except for Potassium Iodide, emphasizing the comparison betweenPotassium Iodide and the reference set of crown/flint combinations.

DETAILED DESCRIPTION

FIG. 1 illustrates a ray trace diagram for spectrometer 10, configuredto disperse incident electromagnetic radiation 20 for imaging,measurement, or other spectral analysis. Spectrometer 10 may be utilizedin any environment, including space-based operations, air-basedoperations, and ground-based operations. Electromagnetic radiation 20received by spectrometer 10 (i.e. electromagnetic radiation 20 emittedor reflected by a target for spectrometer 10) may additionally be fromany location, including space, air, or ground, regardless of theenvironment for spectrometer 10 itself.

Spectrometer 10 may comprise fore-optics, which are not shown in FIG. 1,that form pupil 30. Pupil 30 may be formed from any optical arrangementconfigured to focus electromagnetic radiation 20 a at slit 40. Asnon-limiting examples, the fore-optics may include lenses, mirrors,and/or filters. Where the fore-optics comprises a telescope, thearrangement of the fore-optics may be of any suitable configuration,including but not limited to refractor telescopes (dioptrics), reflectortelescopes (catoptrics), or combinations thereof (catadioptrics). Anynumber of optics arrangements may further be used, including but notlimited to Galilean, Newtonian, or Cassegrain configurations. In anembodiment, the fore-optics may form pupil 30 to direct electromagneticradiation 20 (as electromagnetic radiation 20 a) onto slit 40, so thatslit 40 may take a slice of the image through pupil 30 for spectralanalysis.

Once electromagnetic radiation 20 passes through slit 40 (aselectromagnetic radiation 20 b), it may diverge onto optics 50 forspectrometer 10. Optics 50 may be of any suitable configuration suitableto direct electromagnetic radiation 20 through spectrometer 10, at leastthrough prism 60, described in greater detail below. Optics 50 can be ofany construction or configuration, including, in an embodiment,comprising a double pass collimating and imaging optical system. In somenon-limiting embodiments, optics 50 may generally correspond to thosetaught in U.S. Pat. No. 5,260,767, incorporated in its entirety hereinby reference. As shown in the illustrated embodiment, optics 50 may forma reflective triplet, and thus may include primary mirror 70, secondarymirror 80, and tertiary mirror 90.

In an embodiment, such as that shown, primary mirror 70 may be apositive powered mirror, which may have a conic or higher order asphericconfiguration. As shown, primary mirror 70 may be configured to receiveelectromagnetic radiation 20 b diverging from slit 40, and redirect theelectromagnetic radiation 20 (as electromagnetic radiation 20 c) ontosecondary mirror 80. Secondary mirror 80 may be a negative poweredmirror, positioned substantially on-axis to receive electromagneticradiation 20 c from primary mirror 70, and reflect the electromagneticradiation 20 (as electromagnetic radiation 20 d) onto tertiary mirror90. Tertiary mirror 90 may be positioned to redirect electromagneticradiation 20 (as electromagnetic radiation 20 e) from secondary mirror80 through prism 60, whereby it is dispersed into spectra 100. In anembodiment, the combination of primary mirror 70, secondary mirror 80,and tertiary mirror 90 may be configured to collimate electromagneticradiation 20, such that rays of electromagnetic radiation 20 areparallel as they move out of optics 50 towards prism 60.

As depicted in FIG. 1, prism 60 is a two-material prism having firstprism element 110 and second prism element 120. The shapes of firstprism element 110 and second prism element 110 may vary in differentembodiments, and, in some cases, may be optimized based on thecomputations of a ray trace program. In contrast to the glassy oramorphous nature of conventional prism materials, one or both of firstprism element 110 and second prism element 120 comprise materials thatare non-amorphous and crystalline in their configuration. In anembodiment, first prism element 110 comprises a crystalline crownmaterial. The crown material of first prism element 110 may be anynumber of suitable crowns in accordance with the present disclosure,configured to disperse electromagnetic radiation 20 (i.e.electromagnetic radiation 20 e received by prism 60 from tertiary mirror90). In various embodiments, first prism element 110 may be formed fromor comprise a crystalline crown such as Sapphire (i.e. Aluminum Oxide,or Al₂O₃), Spinel (i.e. Magnesium Aluminum Oxide, or MgAl₂O₄), AluminumOxynitride (AlON), or Magnesium Oxide (MgO). Second prism element 120comprises a crystalline flint material, which may counteract excessiverelative dispersion of electromagnetic radiation 20 by the crownmaterial of first prism element 110 at certain wavelengths, as will bedescribed in greater detail below. In various embodiments, second prismelement 120 may be formed from or comprise a flint such as Zinc Sulfide(in particular water-free Zinc Sulfide, also known as clear ZincSulfide, or ClZnS, such as that marketed as CLEARTRAN™), StrontiumTitanate (SrTi), or Silver Chloride (AgCl).

The effect of electromagnetic radiation 20 passing through first prismelement 110 and second prism element 120 is described in greater detailbelow. In general terms, however, the direction of first prism element110 and second prism element 120 will oppose one another, such that thedispersion of electromagnetic radiation 20 into spectra 100 by firstprism element 110 will be counteracted by second prism element 120. Asshown in FIG. 1, first prism element 110 may be a stronger prism byangle, while second prism element 120 may be a weaker prism by angle. Inan embodiment, such as that illustrated in FIG. 1, first prism element110 is weaker in terms of dispersion (i.e., by being a crown, would havean Abbe number approximately greater than 50), while second prismelement 120 is stronger in terms of dispersion (i.e., by being a flint,would have an Abbe number approximately less than 50). In otherembodiments, depending on the optical arrangement and configuration ofspectrometer 10, first prism element 110 may be a flint prism that isweaker by angle, while second prism element 120 may be a crown prismthat is stronger by angle. In context of the crowns and flints describedabove, Sapphire, Spinel, Aluminum Oxynitride, and Magnesium Oxide, ascrowns, have Abbe numbers between approximately 70 and 55, while waterclear Zinc Sulfide, Strontium Titanate, and Silver Chloride, as flints,have Abbe numbers below 25.

First prism element 110 and second prism element 120 may be joined byany suitable method or configuration. For example, in an embodiment,first prism element 110 may directly contact second prism element 120,such that intermolecular forces hold their contacting portions together(i.e. an optical contact bond). In other embodiments, the elements maybe bonded together by any sort of transparent bonding agent. In someembodiments, there may be a minute air-space between first prism element110 and second prism element 120. For example, in an embodiment, firstprism element 110 and second prism element 120 may be separated by anair space of less than approximately 25-50 μm. In an embodiment where,for example, there is an air or bonding material transition betweenfirst prism element 110 and second prism element 120, the contactingsurfaces of first prism element 110 and second prism element 120 may becoated in an anti-reflective coating, which may prevent undesirablesurface reflections, such as Fresnel reflections, for example, betweenthe elements of prism 60. In some embodiments, such anti-reflectivecoatings may have a thickness on the order of approximately a quarter ofthe wavelength of electromagnetic radiation 20 (including but notlimited to approximately a quarter of the largest wavelength ofelectromagnetic radiation 20 that spectrometer 10 is configured toreceive, approximately a quarter of the average wavelength ofelectromagnetic radiation 20 that spectrometer 10 is configured toreceive, or approximately a quarter of the smallest wavelength ofelectromagnetic radiation 20 that spectrometer 10 is configured toreceive).

In the embodiment of FIG. 1, once electromagnetic radiation 20 eintercepts first prism element 110, it disperses into spectra 100 a.Spectra 100 a then intercepts second prism element 120, and is dispersedagain as spectra 100 b. In the illustrated embodiment, spectra 100 bundergoes a further material-to-air interface once it exits second prismelement 120 as spectra 100 c. As the illustrated embodiment depicts,spectrometer 10 further has flat mirror 130 positioned behind prism 60,such that spectra 100 c leaving second prism element 120 is reflectedback through prism 60 as spectra 100 d. In such an embodiment, optics 50may be optimized for a double pass configuration. In an embodiment,spectra 100 d that is reflected off flat mirror 130 re-enters secondprism element 120 as spectra 100 e, undergoing further dispersion.Additional dispersion occurs when spectra 100 e re-enters first prismelement 110 as spectra 100 f.

Once spectra 100 exits first prism element 110 in the double passconfiguration for spectrometer 10 (as spectra 100 g), it may bereflected back through optics 50. For example, in the illustratedembodiment, wherein optics 50 are optimized for a double passconfiguration, optics 50 may be further configured to redirect spectra100 to detector 140 of spectrometer 10. Although detector 140 is notshown in FIG. 1 in detail, it may be of any construction orconfiguration, including but not limited to a focal plane array, alinear array, a photographic plate, a charge coupled device, a CMOS, orany other element configured to receive and detect spectra 100. In theillustrated embodiment, spectra 100 g is reflected off of tertiarymirror 90 (as spectra 100 h), secondary mirror 80 (as spectra 100 i) andprimary mirror 70 (as spectra 100 j) prior to intercepting detector 140.In an embodiment, detector 140 may be positioned in the same plane asslit 40. In an embodiment, the optics 50 in the reflective tripletconfiguration reform images of slit 40 offset from slit 40, dispersedaccording to wavelength.

As may be understood from the view of FIG. 1, spectra 100 (i.e. spectra100 g leaving prism 60 towards detector 140) must be dispersed at anangle different from that of incident electromagnetic radiation 20 (i.e.electromagnetic radiation 20 e leaving tertiary mirror 90 towards prism60). If any wavelength of spectra 100 is not dispersed, and thus theangle of incidence onto prism 60 equals the angle of transmission fromprism 60 in the double pass configuration, then that wavelength wouldnot be imaged on detector 140, as it would be reflected back onto slit40. In an embodiment, detector 140 may be spaced to either side of slit40, depending on a positive or negative dispersion of spectra 100 forprism 60, and the tilt of mirror 130.

The data in the table of FIG. 2 provides an example of an opticalprescription for spectrometer 10 in FIG. 1. It is to be understood,however, that the spectrometer 10 is not limited to the configurationdescribed in the prescription of FIG. 2, and that the prescription ofFIG. 2 provides details for only one possible embodiment of spectrometer10. Other prescriptions describing the spacings between andcharacteristics of primary mirror 70, secondary mirror 80, and tertiarymirror 90, as well as the materials and surface angles of first prismelement 110 and second prism element 120 may also or alternatively beutilized.

An enlargement of an embodiment of prism 60 is shown schematically inFIG. 3. Although the shape of prism 60 is modified for clarity, it canbe seen that electromagnetic radiation 20 e disperses into spectra 100 aupon impacting first face 150 a of first prism element 110. Furtherdivergence of the angle of spectra 100 a occurs again as spectra 100 acrosses out of second face 150 b of first prism element 110, into firstface 160 a of second prism element 120, as spectra 100 b. In theillustrated embodiment, spectra 100 b may undergo additional divergenceas spectra 100 b exits second face 160 b of second prism element 120, asspectra 100 c, wherein it may impact flat mirror 130. The spacingbetween second prism element 120 and flat mirror 130 may placespectrometer 10 in a minimum deviation configuration. Such spacingbetween flat mirror 130 and prism 60 has the benefit of allowing a tiltto be applied to flat mirror 130, which may be useful in fine tuning thereturn angle of spectra 100 c (as spectra 100 d), allowing for flatterdispersion over a target band of wavelengths, as is discussed in greaterdetail below. A downside to this spaced flat mirror 130 configuration isthe presence of four air-to-material interfaces for prism 60, which mayreduce transmission by some amount as compared to a rear surfacereflection configuration, described below. The additionalair-to-material interfaces can be seen as spectra 100 d, reflected offof flat mirror 130 travels through the air to intercept second face 160b of second prism element 120, traversing through second prism element120 as spectra 100 e and first prism element 110 as spectra 100 f(through first face 160 a of second prism element 120 and second face150 b of first prism element 110), before re-entering the air again asit exits first face 150 a of first prism element 110 as spectra 100 g.

When prism 60 and flat mirror 130 are spaced such that spectrometer 10is approximately in a minimum deviation configuration, the distance fromprism 60 to flat mirror 130 may be arbitrarily established. By minimumdeviation, it may be understood that the orientation (by angle) of prism60 is configured to minimize the total angular deviation between theelectromagnetic radiation 20 and/or spectra 100 as it enters and exitsthe prism 60. Put another way, the entering ray angle (relative to theentrance face surface normal) is approximately equal to the exiting rayangle (relative to the exiting face surface normal). In yet other words,the ray bending occurring at the entrance face of prism 60 isapproximately equal to the ray bending at the exit face of prism 60. Inan embodiment, flat mirror 130 may be spaced as close as possible toprism 60 without their respective mountings interfering with oneanother, or other elements of spectrometer 10. While the distancebetween prism 60 and flat mirror 130 does not generally affect theoperation of spectrometer 10, the tilt of flat mirror 130 affects thefinal location of spectra 100. Additionally, the angles and tilts ofprism 60, affect both the final dispersion and location of spectra 100.Therefore, in an embodiment, both the angles and tilts of prism 60, andthe tilt of flat mirror 130, may be optimized to obtain a desireddispersion, dispersion flatness, and final location for spectra 100.

In an alternative embodiment of spectrometer 10, flat mirror 130 may beomitted, and a portion of prism 60 may provide the reflection for thedouble pass system of optics 50. As shown in the schematic view of FIG.4, second face 160 b of second prism element 120 may be mirrored (i.e.coated with a reflective material, such as a metal layer or amulti-layer dielectric coating) to provide a rear surface reflectionconfiguration for prim 60. While such a configuration may provide a lessflat dispersion over the target band of wavelengths, as discussed ingreater detail below, the configuration has the benefit of having onlytwo air-solid interfaces, which may result in higher overalltransmission efficiency for spectra 100. As seen in the figure,electromagnetic radiation 20 e intercepts first face 150 a of firstprism element 110, and traverses first prism element 110 as spectra 100a. After crossing through second face 150 b of first prism element 110and first face 160 a of second prism element 120, spectra 100 b reflectsfrom mirrored second face 160 b of second prism element 120, immediatelyreflecting back through second prism element 120 as spectra 100 e. Assuch, there is no air-to-material transition occurring such as wouldoccur if spectra 100 b exited through second face 160 b, only tosubsequently re-enter following a subsequent reflection. Spectra 100 ewould then cross first side 160 a and second side 150 b of second prismelement 120 and first prism element 110 respectively, before traversingfirst prism element 110 as spectra 100 f, and exiting first side 150 aas spectra 100 g, making the second air-solid transition of therear-surface reflection configuration of prism 60.

As noted above, the material selection for both the crown of first prismelement 110 and the flint of second prism element 120 plays an importantrole in maintaining a flat relative dispersion (i.e. separation of thevarious wavelengths) of spectra 100 through a desired wavelength band.In some embodiments, the crowns and flints may exhibit characteristicsthat distinguish them from common optical materials, which are typicallyamorphous or glassy. For example, as is plotted in FIG. 5, the materialsdescribed above may generally be outside the range of common opticalglasses such as SF1, in terms of Abbe number verses refractive index.The selection of such materials may have a beneficial effect on therelative dispersion of electromagnetic radiation dispersed therethrough.The relative dispersion of spectra 100 may be characterized as theseparation at an image of spectra 100 between two wavelengths separatedby 10 nm. In the plots of FIGS. 6A-C through FIG. 7, and FIG. 9, therelative dispersion on the Y-axis may be characterized as having theunits “cm/10 nm”, indicating spatial separation in cm per 10 nm ofwavelength separation.

If a spectrometer is configured to observe spectra in the visible andshort-wave infrared wavelength bands (i.e. is a hyperspectralinstrument), some materials or material pairs may work well in the shortwave infrared, however they may provide excessive dispersion in thevisible portion of the spectrum. For example, as seen in FIG. 6A, whichplots relative dispersion compared to wavelength for a variety ofmaterial pairs, while AlON by itself shows a drastic change indispersion as the wavelength drops below 1 μm, the relative dispersionis relatively constant for wavelengths above 1 μm. Such changes inrelative dispersion below 1 μm may be countered by pairing crownmaterials with flint materials, as evidenced by the dispersion plot forthe material pair of AlON with AgCl.

In the enlarged view of FIG. 6B, which re-plots the data of FIG. 6A in acloser scale by omitting AlON by itself, it is seen that countering thedispersion of the first material in first prism element 110 with asecond material in second prism element 120 may result in too muchdispersion in the other direction. As an example, the plot of Spinel andClZnS (i.e. water free or “clear” Zinc Sulfide) is shown to have arelatively large change in dispersion between approximately 0.4 μm and0.5 μm due to this combination. On the other hand, combinations such asSpinel and AgCl maintain a flatter dispersion curve across the full 0.4μm to 2.5 μam spectral range, representing the visible throughshort-wave infrared wavelength bands. By having a flatter dispersion,either fewer pixels on detector 140 are required to fully record therange of wavelengths at a specified average spectral bandwidth, or theaverage spectral bandwidth is reduced given a fixed number of pixels ondetector 140. In both cases, performance of spectrometer 10 is improved.In some cases, the flatness of the dispersion may be characterized bythe maximum relative dispersion over the preferential wavelength bands,as compared to the minimum relative dispersion over the preferentialwavelength bands. For example, in FIG. 6B, the combination ofSpinel/AgCl has a maximum relative dispersion (at 2.5 μm) ofapproximately 0.006, while its minimum relative dispersion (at 0.9 μm)is approximately 0.003, providing flatness of approximately 2:1. Thiswould be superior to material pairs such as Spinel/ClZnS material pair,which has a maximum relative dispersion of 0.0064 and a minimum relativedispersion of 0.0012, leading to a flatness of approximately 5.3:1.

In FIG. 6C the data of FIG. 6B is replotted, omitting those materialpairs that do not utilize Silver Chloride as the flint material. As canbe appreciated from the Figure, the utilization of Silver Chloride asthe flint material yields similar levels of relative dispersion as afactor of wavelength of light for all crown materials shown. FIGS. 7Aand 7B show relative dispersion versus wavelength for a slightlydifferent configuration of two-material prism, in a number ofcrown/flint pairs. By different configuration, it is understood that theangles and weighting of the optical elements may be modified, so as toprovide optimization for a different arrangement and mounting of theoptical elements. In some embodiments, these modifications may compriseone or more of altering the angles of the outer faces of first prismelement 110 and/or second prism element 120 with respect to theircontacting faces, or modifying the angle or orientation of prism 60relative to the entering and exiting electromagnetic radiation20/spectra 100. In some cases, the dispersion in the shorter wavelengthregion (i.e. around approximately 0.4 μm) may be more greatly altered bythese modifications, however they may also alter the location of thepeak dispersion to some degree. FIG. 7A depicts Spinel, AlON, MgO andSapphire crowns in combination with Silver Chloride as the flint. FIG.7B shows those crowns in combination with SrTi and water clear ZnS(ClZnS). In comparing the utilization of material pairs having SilverChloride as the flint material versus the other material pairs, it isapparent that Silver Chloride is an excellent choice for the flintmaterial in two-material prism spectrometers, and may provide a flatterdispersion over a range of wavelengths. As previously indicated, and asis shown in greater detail in the graph of FIG. 8, having a flatdispersion over a wide wavelength band may correlate to less spectralbandwidth at detector 140. In particular, FIG. 8 depicts the bandwidthverses wavelength for two embodiments utilizing the material pair ofSpinel/AgCl, as compared to Infrasil/SF1, a conventional (i.e.glassy/amorphous) material pair. In an embodiment, the dispersion peakfor the prism may be optimized or tuned, such as through ray tracingcomputer simulations, to provide different optimal dispersion effects,which may be useful depending on different variations of the opticalarrangement and mountings in spectrometer 10, or for different detectors140.

As is shown in the graph of FIG. 9, relative dispersion may varydepending on the configuration of prism 60 in terms of whether flatmirror 130 is providing reflection spaced from prism 60, allowing thetilt of flat mirror 130 to permit a minimum deviation configuration, orwhether second face 160 b of second prism element 120 is mirrored, suchthat the “tilt” of the reflecting surface is equal to the angle ofsecond face 160 b. As may be appreciated, the ability to optimize thereflective surface of flat mirror 130 in the minimum deviation (MD)configuration of the Spinel/AgCl material pair generally provides a muchflatter dispersion than the rear surface (RS) reflection configuration.

It may be appreciated that in some embodiments ionic compoundcrystalline salts may be utilized as a flint material, which incombination with a crown material may provide increased flatness indispersion across a wavelength band when used as a two-materialachromatic disperser element. Although in some embodiments, the use ofionic compounds as an optic element may pose manufacturing difficulties,it is possible to mitigate these difficulties so as to achieve theincreased flatness. As an example of such manufacturing difficulties,ionic salts are typically soluble in water. In some cases, thesolubility is sufficiently great that atmospheric humidity may affectthe ability of the ionic compound crystalline salt to be properly shapedinto the optic element. As such, forming and polishing an ionic compoundprism element may require a humidity controlled environment, as anexcess of water in the air during manufacturing may result in the ioniccompound optical element to begin dissolving. In some embodiments, oncethe optic element is formed, its placement in an optical system mayfurther be difficult, as the optics path may desire further protectionfrom outside elements that may harm the use of the dispersing systemutilizing the ionic compound optical element. In some embodiments,however, the optical elements may be utilized in environments that arenaturally devoid of humidity, including, for example, in space-basedoptical platforms, which may make the added expense of protecting theoptics from environmental manufacturing difficulties negligible to thebenefit of their use in the system.

In FIG. 10, the chart of FIG. 5 is repeated, showing Abbe numbers versusrefractive index for the crowns and flints described above in relationto the range of common optical glasses. Added to the chart of FIG. 10,however, are the positions of a variety of ionic compound crystallinesalts, which may be utilized in various embodiments as flint materialsin a crown/flint pair. As seen in FIG. 10, the illustrated ioniccompound crystalline salts, namely Cesium Bromide, Cesium Iodide, andPotassium Iodide, each have a relatively low Abbe number, which suggeststhe flint-like nature of their optical properties. Although CesiumBromide is located in the range of common optical glasses based on itsAbbe number and refractive index, it should be understood that theillustrated range pertains to the dispersive properties of the commonoptical glasses, and not to the crystalline or amorphous (i.e. glassy)properties of the materials in that range.

FIG. 11 shows that when combined with an appropriate crown material,some ionic compound crystalline salts may demonstrate similar (or evensuperior) flatness of relative dispersion across a wavelength band thancomparable substantially crystalline materials that are not ioniccompounds. For example, in the chart of FIG. 11 is the relativedispersion plotted versus wavelength for Cesium Bromide, Cesium Iodide,and Potassium Iodide, utilized in a two-material prism with eitherSpinel or AlON as the crown material, for another embodiment of thespectrometer. Also depicted for use as a reference are the two-materialcombinations of Spinel and AlON with AgCl as the flint, for the samespectrometer. Although different spectrometers may modify the scale ofthe relative dispersions, by plotting the relative dispersions for astandard set of optics, where the only difference is the flints in thetwo-material prisms, a useful relative comparison may be made.

In some embodiments, the general flatness of the relative dispersion ofa two-material combination across a given wavelength band may becharacterized by a merit factor established by the highest and lowestvalue of relative dispersion across the wavelength band, such as theratio of the highest relative dispersion to the lowest relativedispersion. In some embodiments, a merit factor of less thanapproximately 2 may indicate a generally superior combination withregard to flatness across the wavelength band. As may be appreciated inFIG. 11, the maximal relative dispersion for the Spinel/AgCltwo-material pair is approximately 0.003617, while the minimal relativedispersion is approximately 0.001769. Likewise, the maximal relativedispersion for the AlON/AgCl two-material pair is approximately0.003696, while the minimal relative dispersion is approximately0.0017298. As such, the merit factor for the reference combination ofSpinel/AgCL is approximately (0.003617/0.001769), or ˜2.045, while themerit factor for the reference combination of AlON/AgCl is approximately(0.003696/0.0017298), or ˜2.137. Utilizing these merit factors as abaseline for comparison, merit factors for the ionic salt flintcombinations may be calculated.

FIG. 12 shows the graph of FIG. 11, however showing only the referencecombinations of AlON and Spinel with Silver Chloride, alongside thetwo-material combinations of Spinel with Cesium Bromide, and AlON withCesium Bromide. As may be appreciated from FIG. 12, the merit factor forSpinel and Cesium Bromide may be computed as (0.003709/0.001687), or˜2.199. Likewise, the merit factor for AlON and Cesium bromide may becalculated as (0.003953/0.001592), or ˜2.483. Although these meritfactors demonstrate that the dispersion curves are not as flat as AlONand Spinel across the entirety of wavelengths from 0.4 μm to 2.5 μm,combinations utilizing Cesium Bromide may still be a useful and improvedsolution over conventional two-material combinations.

FIG. 13 likewise shows the graph of FIG. 11, however with only CesiumIodide being utilized as the flint in combination with Spinel and AlON,plotted against the reference combinations utilizing AgCl. From theplot, it may be appreciated that the merit factor for Spinel and CesiumIodide is (0.003473/0.001756), or ˜1.978. Likewise the merit factor forAlON and Cesium Iodide may be calculated as (0.003581/0.001699), or˜2.107. As such, it may be appreciated that Cesium Iodide is slightlysuperior to AgCl as a flint material.

Furthermore, in FIG. 14, the graph of FIG. 11 is repeated, with onlyPotassium Iodide being utilized as the flint material in combinationwith Spinel and AlON, plotted against the reference combinationsutilizing AgCl as the flint material. From the plot, it may beappreciated that the merit factor for Spinel and Potassium Iodide may becomputed as (0.003425/0.001778), or ˜1.926. Likewise, the merit factorfor AlON and Potassium Iodide may be computed as (0.003518/0.001739), or˜2.023. As seen in the improved flatness of relative dispersion overwavelengths from 0.4 μm to 2.5 μm compared to previous embodimentsabove, the benefits of utilizing Potassium Iodide as a flint material intwo-material prism elements are evident.

While certain embodiments have been shown and described, it is evidentthat variations and modifications are possible that are within thespirit and scope of the inventive concept as represented by thefollowing claims. For example, in some embodiments, a third material(i.e. either another crown or another flint material) may be utilized inprism 60, and may further flatten the relative dispersion over a desiredwavelength band. The disclosed embodiments have been provided solely toillustrate the principles of the inventive concept and should not beconsidered limiting in any way.

1. A spectrometer comprising: a detector array; a prism comprising: afirst prism element comprising a substantially crystalline crownmaterial; and a second prism element contacting the first prism element,the second prism element comprising a substantially crystalline flintmaterial; and, optics configured to direct light at least twice throughthe prism; wherein the prism is configured to disperse light receivedfrom the optics at an incident angle therethrough into a constituentspectra in visible and infrared wavelength bands, the spectra beingdispersed from the prism at angles offset from the incident angle;wherein the constituent spectra are directed onto the detector arraywith approximately equal dispersion across the visible and infraredwavelength bands.
 2. The spectrometer of claim 1, further comprising amirror configured such that the constituent spectra leaving the prismare reflected by said mirror back through said prism.
 3. Thespectrometer of claim 2, wherein said mirror is spaced apart from theprism.
 4. The spectrometer of claim 2, wherein said mirror is in contactwith a face of the prism to perform rear surface reflection within theprism.
 5. The spectrometer of claim 2, wherein the optics are arrangedin a double pass configuration, such that the constituent spectrareflected by said mirror are reflected through said optics onto thedetector.
 6. The spectrometer of claim 1, wherein the crown material andthe flint material are selected to achieve a relative dispersion for theprism of between approximately 0.001and 0.008 across a wavelength bandof between approximately 0.4 μm and 2.5 μm, so as to provide adispersion characteristic suitable for operation in a visible toshort-wave infrared wavelength band.
 7. The spectrometer of claim 6,wherein the crown material and the flint material are selected toachieve a relative dispersion for the prism of between approximately0.0025 and 0.006 across a wavelength band of between approximately 0.4μm and 2.5 μm, so as to provide a dispersion characteristic suitable foroperation in a visible to short-wave infrared wavelength band.
 8. Thespectrometer of claim 1, wherein the substantially crystalline crownmaterial and the substantially crystalline flint material respectivelydefine a crown/flint pair, the crown/flint pair being selected from thegroup consisting of Al₂O₃ and ZnS, MgAl₂O₄ and SrTi, MgAl₂O₄ and ZnS,AlON and AgCl, MgO and AgCl, MgAl₂O₄ and AgCl, or Al₂O₃and AgCl.
 9. Thespectrometer of claim 1, wherein the substantially crystalline crownmaterial comprises Aluminum Oxynitride, and the substantiallycrystalline flint material comprises Strontium Titanate.
 10. Thespectrometer of claim 1, wherein the substantially crystalline flintmaterial comprises Silver Chloride.
 11. The spectrometer of claim 10,wherein the substantially crystalline crown material comprises Spinel.12. The spectrometer of claim 1, wherein the substantially crystallineflint material comprises an ionic compound crystalline salt.
 13. Thespectrometer of claim 12, wherein the ionic compound crystalline saltcomprises Potassium Iodide, Cesium Iodide, or Cesium Bromide.
 14. Atwo-material prism comprising: a first prism element comprising AluminumOxynitride; and a second prism element comprising Strontium Titanate;wherein an associated face of the first prism element is contacting anassociated face of the second prism element, such that light dispersedthrough both the first prism element and the second prism element has adispersive characteristic suitable for operation in a visible toshort-wave infrared wavelength band.
 15. The two-material prism of claim12, wherein a relative dispersion of the dispersed light is between0.001 and 0.007 across a wavelength band of 0.4 μm to 2.5 μm.
 16. Aspectrometer comprising the two-material prism of claim 12, wherein thetwo-material prism is configured to disperse electromagnetic radiationreceived by the prism into a spectra, and wherein the spectrometerfurther comprises a detector configured to receive said spectra.
 17. Atwo-material prism comprising: a first prism element comprising a crownmaterial; and a second prism element comprising an iodine-based ioniccompound crystalline salt flint material; wherein an associated face ofthe first prism element is contacting an associated face of the secondprism element, such that light dispersed through both the first prismelement and the second prism element has a dispersive characteristicsuitable for operation in a visible to short-wave infrared wavelengthband;
 18. The two material prism of claim 17, wherein the iodine-basedionic compound crystalline salt flint material comprises PotassiumIodide or Cesium Iodide.
 19. The two material prism of claim 17, whereinthe crown material comprises Spinel or Aluminum Oxynitride.